1. Field of the Invention
This invention relates to mobile radio telecommunication transceivers and it relates in particular to a circuit for tuning a transceiver to the strongest of plural receivable frequency channel signals.
2. Prior Art
One prior type of mobile radio telecommunication transceiver is indicated in an A. E. Joel U.S. Pat. No. 3,663,762 which deals with a high capacity telecommunication system. That system is designed to increase the utilization of the available frequency spectrum by employing plural base stations each serving a different part, or cell, of a common geographical region. The base stations are served by a common land line switching office. Within at least one group of base stations, each base station has a different frequency channel for limited supervisory communication with mobile units in its cell. That channel is sometimes also called a data channel, setup channel, or an access channel. The latter term will usually be employed here. Each base station also has a different set of message communication channels that can be employed by mobile units in its cell. In order to facilitate efficient utilization of message communication channels available in the region at any given time, it is desirable that each mobile unit utilize the access channel of the cell in which the unit is then located. To this end the Joel patent has indicated that the mobile unit should have circuits for examining the various receivable access channels and seizing the one of those channels that is the strongest. The presumption underlying this technique is that the base station corresponding to that strongest access channel will be the base station of the cell in which the mobile unit is located. Having once tuned to the strongest access channel, the mobile unit can then be directed by the base station to tune to any available one of the message channels in the cell for establishing and conducting communication between human subscribers.
The foregoing technique for scanning channels and seizing the strongest channel is to be distinguished from prior systems wherein a mobile unit may have plural receivable message channels and it is arranged to scan those channels until it finds one having satisfactory received signal characteristics vis-a-vis the one having the strongest received signals in the entire group of receivable channels.
Present circuits for scanning channels and seizing the strongest thereof employ digital arrangements which, although convenient to manufacture, require substantial hardware. Some such circuits also require relatively critical component design, and all of these circuits require substantial space in a unit that should be as small as possible in order to facilitate portability and to facilitate mounting in a confined vehicle space.